Humans need energy to live in society. Indeed, various forms of energy are used in daily life. Electrical energy is a typical example among those forms of energy. To those who consume electrical energy, the way to use electricity (consume power) considerably fluctuates even during a day. To those who generate electricity, on the other hand, continuous operation of a large-scale power station at a constant level is efficient, and it is difficult to synchronize at short intervals with the small daily fluctuations in power consumption.
In addition to the above, various methods of using natural energy such as wind power, sunlight, wave power, and geothermal energy are under discussion and consideration, but often, electricity generation by natural energy does not match the needs of a human society.
In order to fill the gap described above, techniques of storing energy are required. The “CAES” is one of such methods.
The technique CAES is a system in which power generated during midnight at a gas turbine power station is used to store high-pressure air into a reservoir, for example, an underground cavern or a seabed tank in the form of pressure potential, and electricity is generated by use of the stored high-pressure air when the power consumption is high during the daytime.
Regarding the electricity generation systems using compressed air energy storage, reports are presented by the Central Research Institute of Electric Power Industry (incorporated foundation) (see Non-patent Literatures 1 and 2).                Compressed air energy storage (CAES) is implemented in Germany and the United States.        In these countries, compressed air is stored underground in halite layers.        In Japan, which has no halite layers, a possible way is to dig an underground cavern or to install a storage tank on the seabed.        The efficiency of the CAES is currently inferior to that of superconductivity but is superior to that of pumped-storage hydroelectricity. Technological advancements in the future may bring the efficiency of the CAES near the level of superconductivity.        At the current level of technology, it is possible to dig out a cavern for the CAES in hard rock, but there are problems with digging out soft rock and installing a seabed tank.        The CAES for a cavern in hard rock is, under favorable conditions, economically competitive against the pumped-storage hydroelectricity and LNG-combined cycle electricity generation.        Other storage methods require technological development which enhances economic efficiency.        If all fossil-fuel power stations were to be replaced with CAES's, 55% of the current amount of fuel consumption could be saved.        If the CAES could be implemented in the underground soft rock of the suburbs of a large city, problems as with installation of power cables would be alleviated, and the CAES would provide distributed generation in the city.        As of 1990, programs for compressed air storage gas turbine electricity generation listed countries such as Germany (former West Germany), Italy, the United States, Ukraine (the former Soviet Union), France, Luxembourg, and Japan, and halite layers are often designated. Since Japan has no halite layers, however, digging of a cavern in bedrock is planned to be conducted.        In an example of operation, which is under way in Germany, a cylindrical cavern is formed in a halite layer, and it measures 55 m in diameter and 150 m in length.        The compressed air storage is divided into two, namely, variable-pressure storage and constant-pressure storage, and their storage volumes vary depending on the storage pressure. The constant-pressure storage can reduce the storage volume. In comparison with the pumped-storage hydroelectricity, for example, the pressure potential of air of 80 atm with volume 1 m3 equals the potential energy of water of 1 m3 lifted to the height of 810 m.        In order to achieve, for example, 10 hours of electricity generation at 100,000 kW using an existing gas turbine in the case of the constant-pressure storage, the storage volume necessary for the electricity generation is calculated to 140,000 m3 for storage at 30 atm, and 84,000 m3 for storage at 50 atm.        
As described above, at a constant pressure, it is advantageous to store compressed air at high pressure (high storage pressure) since the storage volume is small. Conversely stated, in a cavern where there is leakage of air, the storage pressure decreases and loss is incurred despite a large amount of civil engineering operation. Some countries create and make practical use of caverns for storage of compressed air in halite layers, but Japan has no halite layers with such favorable conditions, but rather, weak strata of earth with many faults. Hence, it is necessary to develop a technique capable of creating a cavern for the storage of the compressed air without the leakage of air even in such strata of earth.
Given the circumstances, regarding the making of an underground cavern, there has been proposed a method of making a seal layer with a predetermined thickness along an inner wall of a storage space by: placing a flexible water-tight balloon inside the storage space, then providing a filler between the balloon and the storage space and supplying pressurized water inside the balloon, and curing the filler provided between the balloon and the storage space, as a method capable of effectively filling numerous cracks in the inner wall of the storage space (see Patent Literature 1).
Since the underground cavern used in the CAES is very large, however, it is troublesome to place the balloon inside the underground cavern and make the seal layer along the inner wall of the underground cavern which serves as the storage space for the compressed air, not to mention the manufacturing and the preparing of the balloon fitted to the inner shape of the underground cavern.